(1) Field of the Invention
The present invention relates to a method and apparatus for detecting deterioration of a sucked air flow quantity-detecting device used in an electronically controlled fuel injection system of an internal combustion engine.
(2) Description of the Related Art
As an example of a conventional electronically controlled fuel injection system, attention is directed to the system mentioned in Japanese Unexamined Patent Publication No. 60-240840.
Namely, a flow quantity Q of air sucked into an engine is detected based on a voltage signal emitted from a sucked air flow quantity-detecting device such as a flap type flow meter or hot-wire type flow meter, and in a control unit, a basic fuel injection quantity Tp=K.multidot.Q/N, where K is a constant is calculated from this sucked air flow quantity Q and an engine revolution number N calculated from an ignition signal to an ignition coil or a crank angle signal from a crank angle sensor.
Then, this basic fuel injection quantity is corrected by various correction coefficients COEF corresponding to the cooling water temperature and the like and a voltage correction component Ts corresponding to the battery voltage and the final fuel injection quantity Ti=Tp.multidot.COEF+Ts is thus calculated.
A driving pulse signal having a pulse width corresponding to the above-mentioned fuel injection quantity Ti is emitted at a synchronous with the revolution of the engine to drive and open a fuel injection valve and effect injection of fuel.
In the conventional system, during idle driving of the engine, for example, when an idle switch is turned on, an upper limit level I of the sucked air flow quantity Q, which is larger by .DELTA.Q1 than the sucked air flow quantity Q for idle driving and cannot be reached in the idle state, is set, and if the sucked air flow quantity Q exceeds this upper limit level I, it is presumed that a disorder has taken place. When the idle switch is turned off, in the driving region where the engine is not in the idle state, a lower limit level II of the sucked air flow quantity Q, which is smaller by .DELTA.Q2 than the sucked air flow quantity Q for idle driving and below which the sucked air flow quantity Q does not decrease even in the idle state, is set, and if the sucked air flow quantity Q is lower than this lower limit level II, it is presumed that a disorder has taken place in the sucked air flow quantity-detecting device.
In this conventional disorder-detecting method, however, although an absolute disorder of the sucked air flow quantity-detecting device can be detected, an unexpected deterioration which has not reached an absolute trouble condition is a variation included within the ranges of .DELTA.Q1 and .DELTA.Q2 and hence, cannot be detected. This deterioration has serious influences on the exhaust gas-purging performance of a vehicle and it is difficult to maintain good exhaust gas-purging performance of a vehicle for a long time.
Of course, it is considered that an unexpected deterioration will be detected by setting smaller values for .DELTA.Q1 and .DELTA.Q2. However, according to this method, even an inherent dispersion of the sucked air flow quantity-detecting device, a change of the atmospheric pressure irrelevant to the sucked air flow quantity-detecting device, or contamination or clogging of the suction system is erroneously judged as a deterioration condition.